Ceramic gas sensors of this kind have been known for a long time. These gas sensors are preferably utilized in automobiles for detecting toxic exhaust gases and have an ion-conducting ceramic such as zirconium oxide (ZRO.sub.2). The ceramic must be heated in order to ensure a proper operation. For this reason, a heater is embedded in the ceramic. The heater must be insulated because the ceramic is electrically conductive in the heated operational state.
The insulation of the heater is, however, problematic (especially at high temperatures) because leakage resistances result from the heater to the sensor electrodes in the order of magnitude of several megaohms.
This is non-critical in probes having high measuring currents, such as lambda probes. However, for gas sensors wherein the measuring effect lies in the order of magnitude of several microamperes and which gas sensors must be operated in combination with circuits which evaluate the current, a very large measurement error occurs. The leakage current flowing away from the heater is superposed on the actual measuring current and causes relatively large errors in the measurement result.
Large differences occur with respect to individual gas sensors and a very significant deterioration occurs. For this reason, the leakage current cannot be simply compensated by a fixed corrective quantity.
For the above reason, an isolation amplifier has, for example, been used up to now for measuring the measurement current. Because of the isolation amplifier, the actual measuring circuit is driven separately with respect to potential and the measurement quantity is transmitted in an insulated manner for further signal processing. The potential partition can, for example, be achieved with a transformer or optoelectronically. In this case, the voltage potential at the sensor electrodes so adjusts that the leakage current becomes zero because this leakage has nowhere to flow to. The consequence thereof is that only the desired measuring current is measured. It is disadvantageous in this measurement method that there is a high requirement of expensive components which cannot be integrated or can be integrated only with difficulty.
For sensors which supply a voltage signal, it is known to make the measurement via a so-called electrometer or instrument amplifier. This amplifier measures the voltage "without current" so that a leakage current has no effect on the measurement result.
U.S. Pat. No. 5,054,452 discloses a method and an arrangement for detecting a fault condition of a lambda probe and measures are taken as a consequence of a fault signal which is outputted when a fault condition is detected. In this method, impermissibly large fault causes can be diagnosed during operation of the lambda probe utilizing a correlation method. Shunts exist only during operation of the heater. For this reason, the probe heater is switched off in this measurement method for detecting a fault condition because, in this case, no shunt voltage and therefore no falsification of the measurement result is present. The difference between the probe voltage measured for a switched-on heater and measured for a switched-off heater is the shunt voltage.
It is problematic in this method that it cannot be easily applied to a current measurement.